Why Can\'t I Make the `operator

Implementing virtual operator<<

The need for a virtual operator<< arises when customizing the behavior of the streaming operator for different classes. Attempts to define the operator as virtual often lead to the error "operator <<: only member functions and bases can be virtual."

The Dilemma of Free Functions

The issue arises because operator<< is typically defined as a free function, which lacks a receiver object and cannot be virtual. Defining the operator as a member function, on the other hand, reverses the parameter order, causing a compilation error.

A Solution via Indirection

To resolve this dilemma, consider adding a virtual member function that encapsulates the desired output behavior:

<code class="cpp">class MyClass {
public:
    virtual void print(ostream& out) const; // Virtual output function
};</code>

Customized Operator with Virtual Behavior

With the virtual member function in place, you can define the operator<< as a free function that delegates to the print function:

<code class="cpp">ostream& operator<<(ostream& out, const MyClass& mc) {
    mc.print(out); // Calls the virtual print function
    return out;
}</code>

This approach maintains the correct parameter order for the operator<< free function while allowing customization of the output behavior through the virtual member function. Subclasses can override the print function to implement their own custom output formatting.

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